Bolton Wanderers midfielder Fabrice Muamba collapsed and stopped breathing during the FA Cup fifth-round tie against Tottenham.
Medics spent 10 minutes trying to resuscitate the 23-year-old on the field after he fell to the ground with no other players around him.
BBC Radio 5 live's Ian Dennis says the former England Under-21 international is fighting for his life.
The score was 1-1 when the match was abandoned after 41 minutes.
Medical staff gave Muamba mouth-to-mouth resuscitation and tried to revive him with a defibrillator. In total six medics were treating the player.
Both sets of supporters chanted Muamba's name as he was taken off the pitch on a stretcher. ESPN, who were broadcasting the match, reported that he was not breathing as he was taken into the tunnel.
Referee Howard Webb called the visibly shaken players off the field after consulting with Bolton manager Owen Coyle and Tottenham boss Harry Redknapp.
Minutes later the match was abandoned.
A Bolton spokesman said: "Fabrice Muamba has been taken to hospital following his collapse. There is no further information at this time."
The former Arsenal midfielder was accompanied in the ambulance by Coyle and club captain Kevin Davies.
BBC Radio 5 live's chief football reporter Dennis, who was covering the game at White Hart Lane, witnessed medics rushing to aid Muamba.
"When the medical staff arrived his face was on the turf," he said.
"You could see them using a defibrillator and he was [electronically] charged on at least two occasions. Everyone could see Fabrice Muamba was fighting for his life.
"If he does pull through it will be down to the quick actions of the medics."
Soon after the incident, messages of support flooded in for the former Birmingham player.
Spurs player Rafael van der Vaart was one of the players on the pitch when Muamba collapsed. He tweeted: "Terrible what happened with Muamba during the game. We're all praying for him."
England international Jack Wilshere, who played with Muamba at Bolton, tweeted: "Hope Muamba is okay. Thoughts with him."
Middlesbrough defender Justin Hoyte, who played in the Arsenal youth team with Muamba, said: "I seriously hope my best friend in football is OK. Stay strong bro [sic] please please stay strong. God is with you remember that."
Professional Footballers' Association chief executive Gordon Taylor said: "At the moment we are all watching, waiting, hoping and praying. For all the football family it's a terrible thing to see.
"We just hope it's not going to be a situation like that of Marc-Vivian Foe.
"This can happen on a pitch and one of the reasons why we insist all youngsters coming into the game have the ability to have heart screenings to avoid this happening."
Muamba joined Wanderers from Birmingham in 2008 and has made 148 appearances for the club.
********Sorry about Vid - Youtube are removing them all!****

Fabrice is being cared for in heart attack unit in intensive care at London Chest Hospital. He is still in a critical condition.
They are not speculating what happened to him but where he is being treated kind of spells it out.
They are saying that if not for the medical staff that treated him pitch side, he probably would not have come this far.

The Following 2 Users Say Thank You to sarahowen662 For This Useful Post:

Fabrice is being cared for in heart attack unit in intensive care at London Chest Hospital. He is still in a critical condition.
They are not speculating what happened to him but where he is being treated kind of spells it out.
They are saying that if not for the medical staff that treated him pitch side, he probably would not have come this far.

Thanks as I was going to post the update. Absolutely horrible as he was one of if not the most fit player on the team from all accounts.

It makes you wonder if he has a previously undiagnosed heart issue or disease.

This reminds me of Hank Gathers who played basketball in my city (Philadelphia) and collapsed on the court and died. He had a heart issue that was not known previously. I would have thought that after that incident that heart testing would be mandatory for all sports.